WO2010032776A1

WO2010032776A1 - Hot-pressed steel plate member and manufacturing method therefor

Info

Publication number: WO2010032776A1
Application number: PCT/JP2009/066227
Authority: WO
Inventors: 瀬沼　武秀; 吉田　寛
Original assignee: 国立大学法人岡山大学
Priority date: 2008-09-18
Filing date: 2009-09-17
Publication date: 2010-03-25
Also published as: EP2339044A4; JP2010070806A; JP5637342B2; KR20110053474A; US8449700B2; US20110226393A1; CN102232123A; EP2339044A1

Abstract

Disclosed are a high-strength, high-toughness hot-pressed steel plate member and a manufacturing method therefor. A specified hot-press process is performed on a steel plate member which, with respect to the chemical composition of the steel plate, comprises: 0.15-0.4 wt% of C; 1.0-5.0 wt% of Mn or of a total of Mn and at least one of Cr, Mo, Cu, and Ni; 0.02-2.0 wt% of at least Si or Al; and the remainder being FE and unavoidable impurities, thus providing the following physical properties: martensite phase average grain diameter of 5 μm or less, and tensile strength of 1200 MPa or higher.

Description

Hot-pressed steel plate member and method for manufacturing the same

The present invention relates to a hot-pressed steel plate member having a martensitic microstructure and a method for manufacturing the same.

Steel plates are often used for automobiles. Various weight reductions are applied to automobiles in order to improve fuel efficiency. Steel plate members are also targeted for weight reduction. That is, it is required to reduce the thickness and weight by increasing the strength of the steel plate member.

However, steel plate members used in automobiles are often used for members intended to protect passengers in the event of a collision, such as door impact beams and center pillar reinforcements. Therefore, such a steel plate member must be able to reliably maintain a predetermined strength.

In particular, when manufacturing a high-strength steel plate member used in automobiles by using a hot stamping technology, in a general hot stamping technology, the steel plate member is heated above the transformation point and pressed using a mold in the austenite region. While it is molded, it is transformed into martensite by removing heat with a mold.

It is known that a steel plate member having a predetermined shape using a hot stamping technique has a toughness value because it remains in a quenched structure.

Therefore, when it is desired to improve the toughness value, a steel plate member or steel material may be tempered after processing by the hot stamping technique.

Further, by optimizing the composition of the steel material and the heat treatment conditions, a high-tensile cold-rolled steel sheet having a tensile strength of 880 to 1170 MPa as a martensite single-phase structure (see, for example, Patent Document 1), and a space factor of 80% A high-strength steel has been proposed in which the average particle size of the martensite phase is 10 μm or less and the tensile strength is 780 MPa or more (see, for example, Patent Document 2).
Japanese Patent No. 3729108 JP 2008-038247 A

However, in the case of high-tensile cold-rolled steel sheets with a martensite single-phase structure, and high-strength steels with an average particle size of martensite phase with a space factor of 80% or more and 10 μm or less, there are limitations in the examples Thus, it was difficult to make the average particle size 5 μm or less, and it was difficult to ensure toughness with a steel material having a tensile strength exceeding 1200 MPa.

In view of the present situation, the present inventors have conducted research and development to provide a steel member having high strength and high toughness by further reducing the average particle size of the martensite phase, and have reached the present invention.

The steel plate member subjected to the hot press processing of the present invention has a C content of 0.15 to 0.4% by weight in the chemical composition of the steel plate, a Mn content or a total of at least one of Cr, Mo, Cu, Ni and Mn. Is 1.0 to 5.0% by weight, at least one of Si and Al is 0.02 to 2.0% by weight, the balance is Fe and inevitable impurities, and the average particle size of the martensite phase is physical properties It is 5 μm or less, has a tensile strength of 1200 MPa or more, and is achieved by applying a specific hot press process.

Furthermore, the hot-pressed steel sheet member of the present invention is characterized in that the content of at least one of B, Ti, Nb, and Zr is 0.1% by weight or less, and has a thickness of 0.1 to It is also characterized by having a 20 μm plating film.

Further, in the method for producing a hot-pressed steel sheet member according to the present invention, the C content is 0.15 to 0.4% by weight, the Mn content or the total content of at least one of Cr, Mo, Cu, Ni and Mn. A raw steel plate having a chemical composition composed of 1.0 to 5.0% by weight, at least one of Si or Al content of 0.02 to 2.0% by weight and the balance of Fe and inevitable impurities is used. A method of manufacturing a steel sheet member having a physical property of an average martensite phase particle size of 5 μm or less and a tensile strength of 1200 MPa or more by pressing, wherein the hot pressing is 675 at a temperature increase rate of 10 ° C./second or more. Heating process for heating up to a maximum heating temperature T ° C of ~ 950 ° C, temperature holding process for maintaining the maximum heating temperature T ° C in (40-T / 25) seconds or less, and 1.0 ° C / second or more from the maximum heating temperature T ° C Below the Ms point which is the formation temperature of the martensite phase at the cooling rate of It has a cooling process of cooling while pressing.

Furthermore, in the method for manufacturing a steel plate member subjected to hot pressing according to the present invention, the steel plate member contains at least one of B, Ti, Nb, and Zr at a content of 0.1% by weight or less, during the cooling step. It is also characterized by performing press processing to form a steel plate member into a predetermined shape at least once before reaching the Ms point, and performing cold rolling with a rolling rate of 30% or more on the steel plate member before the heating step. Is.

According to the present invention, since the average particle size in the martensite phase can be 5 μm or less, it is possible to provide a high-strength steel plate member having a tensile strength of 1200 MPa or more while improving toughness.

It is a SEM photograph image which photoed the martensitic phase in the steel plate member of experiment number 6. It is a SEM photograph image which image | photographed the martensitic phase in the steel plate member of the experiment number 3 which performed the hot press process of this invention.

In the steel plate member subjected to hot pressing and the manufacturing method thereof according to the present invention, the metal structure of the steel plate member, particularly the average particle size of the martensite phase is 5 μm or less, and the strength is improved while improving toughness. is there. In particular, the steel sheet member of the present invention has a tensile strength of 1200 MPa or more.

Here, the steel plate member is not limited to the case where it is a martensite single phase. It is only necessary that the average particle size of the martensite phase is 5 μm or less in the region that is the martensite phase. In addition, the average particle diameter of a martensite phase is an average value of the crystal particle diameter of a martensite phase.

Such a steel plate member has a C content of 0.15 to 0.4% by weight, an Mn content or a total content of at least one of Cr, Mo, Cu, and Ni and Mn of 1.0 to 5.0% by weight, Si or Al. Is contained in an amount of 0.02 to 2.0% by weight, and the balance is composed of Fe and inevitable impurities.

The steel sheet member was heated to a maximum heating temperature T ° C. of 675 to 950 ° C. at a temperature rising rate of 10 ° C./second or more, and the maximum heating temperature T ° C. was maintained in (40−T / 25) seconds or less. Then, the martensite phase is generated by performing hot press processing which cools while pressing to the Ms point or less which is the formation temperature of the martensite phase at a cooling rate of 1.0 ° C./second or more from the maximum heating temperature T ° C.

Moreover, the average particle size of the martensite phase can be 5 μm or less, and a high strength and high toughness steel material or steel plate member having a tensile strength of 1200 MPa or more can be obtained. Furthermore, the average particle size of the martensite phase can be further reduced by containing at least one of B, Ti, Nb, and Zr in a content of 0.1 wt% or less in the steel plate member.

In the following, it will be described in detail with examples.

First,
C content: 0.22% by weight,
Mn content: 3.0% by weight,
Si content: 0.05% by weight,
Al content: 0.05% by weight,
Ti content: 0.02% by weight,
B content: 0.002% by weight
As described above, a steel plate having a thickness of 1.4 mm was prepared using steel composed of Fe and inevitable impurities as the balance. This steel plate member was cold-rolled at a rolling rate of 60%.

The steel plate members were heated at a maximum temperature T of 650 ° C., 700 ° C., 775 ° C., 850 ° C., 950 ° C. and 1000 ° C. at a rate of temperature increase of 200 ° C./second, respectively. The temperature was maintained for 0.1 second, and then cooled to the Ms point or lower, which is the formation temperature of the martensite phase, at a cooling rate of 10 ° C./second. However, when the maximum temperature T was 1000 ° C., the retention time of the maximum temperature T was 4 seconds. The steel plate member was heated by energization heating, and the steel plate member was cooled by natural cooling.

Further, during cooling from the maximum temperature T to the Ms point or less, the steel plate member was subjected to a hat-type press forming while being lowered by 100 to 150 ° C from the maximum temperature T, and further lowered by 50 to 100 ° C. The steel plate member was punched.

After the steel plate member was sufficiently cooled, test pieces were cut out from the top of the hat-shaped steel plate member, and subjected to a tensile test and a Charpy impact test. In the Charpy impact test, three test pieces were stacked.

Table 1 shows the average particle diameter, tensile strength, and transition temperature of the martensite phase at each maximum temperature T. The transition temperature is an index of toughness, and the lower the toughness, the higher the value.

As shown in Table 1, when the maximum temperature T was 650 ° C., the reverse transformation to the austenite phase did not occur sufficiently, so that the martensite phase was not sufficiently generated, and the average grain size of the structure It is considered that the diameter is large and the transition temperature is high.

On the other hand, when the maximum temperature T is 1000 ° C., the structure becomes coarse and the transition temperature is high. FIG. 1 is an SEM photograph image of the martensite phase in the case of experiment number 6.

From this experimental result, the maximum temperature T is considered to be 675-950 ° C. In addition, after heating at a temperature increase rate of 200 ° C./second with a maximum temperature T of 775 ° C. and holding the maximum temperature T for 1.0 second, each Ms is a martensite phase generation temperature at a cooling rate of 10 ° C./second. The SEM photograph image which image | photographed the martensite phase at the time of cooling to the point or less is shown in FIG. In this case, the average particle size of the martensite phase was 1.7 μm, the tensile strength was 1532 MPa, and the transition temperature was −70 ° C.

Using the steel plate member having the composition of Example 1 described above, the maximum temperature T was set to 800 ° C., and the rate of temperature increase was set to 5 ° C./second, 15 ° C./second, and 200 ° C./second as in Example 1. A test piece was prepared. Each temperature was held for 0.1 seconds at the maximum temperature T, and then cooled to a temperature below the Ms point, which is the martensite phase formation temperature, at a cooling rate of 10 ° C./second.

Table 2 shows the average particle size, tensile strength, and transition temperature of the martensite phase at each rate of temperature increase.

As shown in Table 2, when the rate of temperature increase is 5 ° C./second, the structure of the martensite phase is coarsened and the transition temperature is high.

From this experimental result, it is sufficient that the rate of temperature increase is 10 ° C./second or more. On the other hand, from the result of Experiment No. 5 in Table 1, when the rate of temperature increase is 200 ° C./second and the maximum temperature reached is 950 ° C., the average particle size of the martensite phase is 1.9 μm. In order to achieve this, the heating rate is preferably 200 ° C./second or more. The upper limit of the heating rate depends on the ability of the heating device for heating the steel plate member. However, when the heating device is an energization heating device, heating is performed at 200 ° C./second or more without any problem because high-speed heating is easy. be able to.

Using the steel plate member having the composition of Example 1 described above, the maximum temperature T was 800 ° C., the rate of temperature increase was 200 ° C./second, and the temperature holding time at the maximum temperature T was 0.1 seconds, 2.0 seconds, The test piece similar to Example 1 was produced as second. The steel plate members were each cooled to a temperature below the Ms point, which is the martensite phase formation temperature, at a cooling rate of 10 ° C./second. The test piece with a temperature holding time of 0.1 second is the test piece of Experiment No. 9 in Example 2 described above.

Table 3 shows the average particle size, tensile strength, and transition temperature of the martensite phase at each temperature holding time.

As shown in Table 3, when the temperature holding time is as long as 12 seconds, the structure becomes coarse and the transition temperature is high. That is, it is desirable that the temperature holding time is as short as possible.

In particular, it has been found that the temperature holding time is preferably shorter as the temperature of the maximum temperature T is higher, and is preferably (40−T / 25) seconds or less.

In other words, the temperature holding time is desirably (40−T / 25) seconds or less with respect to the maximum temperature T, and if the steel sheet member cannot be cooled immediately after heating due to the structure of the apparatus, the maximum temperature is reached. It is desirable that a margin be provided by setting T to the lowest possible temperature of 675 to 950 ° C.

Using the steel plate member having the composition of Example 1 described above, the maximum temperature T was 800 ° C., the rate of temperature increase was 200 ° C./second, the temperature holding time at the maximum temperature T was 0.1 seconds, and the steel plate member was 0.5 ° C. A test piece similar to that of Example 1 was manufactured by cooling to the Ms point or less at respective cooling rates of 10 ° C./second, 10 ° C./second, and 80 ° C./second. In addition, the test piece which made the cooling rate 10 degree-C / sec is a test piece in the experiment number 9 of Example 2 mentioned above.

Table 4 shows the average particle size, tensile strength, and transition temperature of the martensite phase at each cooling rate.

As shown in Table 4, when the cooling rate is as low as 0.5 ° C./second, the structure becomes coarse and the transition temperature becomes high. That is, it is desirable that the cooling rate be as fast as possible. In order to increase the cooling rate, the steel plate member may be cooled using a coolant such as water.

However, if the cooling rate is too high, press working for forming the steel plate member into a predetermined shape may not be completed before reaching the Ms point, so about 1.0 to 100 ° C./second is desirable. If possible, the cooling rate may be 100 ° C./second or more.

When pressing a steel sheet member below the Ms point, it tends to cause deterioration of shape freezing property and delayed fracture resistance, so the cooling rate can be determined in consideration of the time required for pressing. desirable.

As long as the temperature of the steel sheet member does not reach the Ms point, the pressing may be performed not only in one step but also in multiple steps. By performing the pressing at a temperature higher than the Ms point, excellent shape freezing property can be obtained. Can do.

In the steel plate member having the composition of Example 1 described above, the cold rolling process was performed at a rolling rate of 60% and the thickness was 1.4 mm. However, when the cold rolling process was not performed, that is, the rolling rate was 0%, A test piece was prepared when the thickness dimension of the member was increased. In the preparation of this test piece, the maximum temperature T was 800 ° C., the temperature increase rate was 200 ° C./second, and the temperature holding time at the maximum temperature T was 0.1 seconds. The cooling rate was 3 ° C./second for a test piece having a rolling rate of 0% and a thickness of 1.4 mm, and 10 ° C./second for a test piece having a rolling rate of 0% and a thickness of 4.2 mm.

Table 5 shows the average particle diameter, tensile strength, and transition temperature of the martensite phase in the test piece.

Thus, it can be seen that the martensite phase is refined and toughened in the steel plate member without performing cold rolling.

However, when cold rolling is not performed, the average particle size of the martensite phase is about 3.0 μm, but as shown in Examples 1 to 4, by performing cold rolling at a rolling rate of 60%, Since the average particle size is about 2.0 μm, the toughness can be improved by cold rolling.

In order to obtain an average particle size of the martensite phase of about 2.0 μm, it is sufficient that the cold rolling process is performed at a rolling rate of about 30%, and the refinement effect is saturated in the high rolling rate region, and the cold rolling process is performed. Since the processing cost increases, the upper limit of the rolling rate is about 95%.

In addition, the thickness of the steel plate member is preferably up to about 5.0 mm in order to perform rapid heating at a heating rate of 50 ° C./second or more as uniformly as possible. A large steel plate member can also be used.

Note that if the steel plate member is thinner than 0.1 mm, deformation may occur during rapid heating at a heating rate of 50 ° C / second or more, so the lower limit is set to 0.1 mm or deformation due to heating is prevented. It is desirable to use an auxiliary jig or the like.

A steel plate member having a thickness of 1.4 mm was produced using the steel types in the composition table shown in Table 6 below. For this steel plate member, the maximum temperature T is 800 ° C., the rate of temperature increase is 200 ° C./second, the temperature holding time at the maximum temperature T is 0.1 seconds, and the steel plate member is Ms point or less at a predetermined cooling rate. The sample was cooled while being pressed until the same test piece as in Example 1 was produced.

In addition, the unit of the composition table is% by weight, and the balance consists of Fe and inevitable impurities.

Table 7 shows the average grain size, tensile strength, and transition temperature of the martensite phase in the test pieces of each of the steel types A to L.

As shown in Table 7, the transition temperature is high in steel type E where C is as high as 0.50% by weight, and conversely, in steel type G where C is as low as 0.10% by weight, the average grain size of martensite grains The diameter is coarse. Moreover, transition temperature is high in steel type H in which Mn is as high as 6.2% by weight.

From this, the steel sheet member has a C content of 0.15 to 0.4% by weight, an Mn content of 1.0 to 5.0% by weight, a content of at least one of Si or Al of 0.02 to 2.0% by weight, the balance being Fe and Inevitable impurities are desirable.

As shown in steel types I to L, the amount of Mn used may be suppressed by substituting a part of Mn with at least one of Cr, Mo, Cu, Ni. Cr, Mo, Cu, Ni The total content of at least one of the above and Mn may be 1.0 to 5.0% by weight.

In addition, Si or Al can be added 0.02% by weight or more to reduce dissolved oxygen and suppress the generation of voids in the steel. On the other hand, if 0.2% by weight or more is added, the average particle size of the martensite phase is reduced. In order to increase the coarseness, the content is preferably 0.02 to 2.0% by weight.

Furthermore, in order to refine the martensite phase, it is desirable to contain at least one of B, Ti, Nb, and Zr. Particularly, when 0.1% by weight or more is added, the refinement effect is saturated. Therefore, it is desirable to make it 0.1% by weight or less.

Such a steel plate member can be provided with a plating film having a thickness of 0.1 to 20 μm to prevent scale from being generated on the surface of the steel plate member using this plating film as a protective film.

As the plating film, a Ni electroplating film, a Cr electroplating film, a hot dip galvanizing film, a hot dip aluminum plating film, or the like can be used, and the required film thickness may be used as necessary. The plating film may be 20 μm or more, but 20 μm or less is sufficient because the protective effect of the plating film is saturated.

As described above, the steel plate member has a C content of 0.15 to 0.4 wt%, a Mn content, or a total content of Mn and at least one of Cr, Mo, Cu, Ni in the chemical composition of the steel plate. 1.0 to 5.0 wt%, the content of at least one of Si or Al is 0.02 to 2.0 wt%, the balance is Fe and unavoidable impurities, and this steel sheet member is 675 to at a heating rate of 10 ° C / sec or more. After heating to 950 ° C maximum heating temperature T ° C and holding the maximum heating temperature T ° C for (40-T / 25) seconds or less, martensite at a cooling rate of 1.0 ° C / second or more from the maximum heating temperature T ° C. By applying a hot press process that cools while pressing to the Ms point or lower, which is the production temperature of the steel sheet, in the physical properties, it can be a steel plate member having a microstructure with an average grain size of martensite grains of 5 μm or less, Moreover, the tensile strength can be 1200 MPa or more.

Furthermore, the steel plate member can be made into a steel plate member or steel material having a microstructure with an average particle size of martensite grains of 2 μm or less by performing cold rolling with a rolling rate of 30% or more in advance, The tensile strength can be 1500 MPa or more.

In addition, since the cooling rate can be reduced to 1.0 ° C / second or more, a steel plate member or steel material can be formed into a predetermined shape by pressing until the Ms point is reached, so high productivity is not lost. Strength and high toughness steel plate members or steel materials can be manufactured.

Claims

The chemical composition of the steel sheet is 0.15 to 0.4% by weight for C content, 1.0% to 5.0% by weight for Mn content or the total content of Mn and at least one of Cr, Mo, Cu, Ni, Si or Al The content of at least one of 0.02 to 2.0% by weight, the balance being Fe and inevitable impurities,
The physical properties are steel plate members subjected to hot pressing with an average particle size of the martensite phase of 5 μm or less and a tensile strength of 1200 MPa or more.
The steel plate member subjected to hot pressing according to claim 1, wherein the content of at least one of B, Ti, Nb, and Zr is 0.1% by weight or less.
3. A steel plate member subjected to hot pressing according to claim 1 or 2, wherein the surface has a plating film having a thickness of 0.1 to 20 μm.
C content is 0.15-0.4% by weight, Mn content or the total content of at least one of Cr, Mo, Cu, Ni and Mn is 1.0-5.0% by weight, and contains at least one of Si or Al Using a raw steel plate having a chemical composition composed of 0.02 to 2.0% by weight, the balance being Fe and inevitable impurities, the physical properties of the raw steel plate are hot-pressed and the average particle size of the martensite phase is 5 μm or less. A method for producing a steel sheet member having a tensile strength of 1200 MPa or more,
Hot press processing
A heating step of heating to a maximum heating temperature T ° C. of 675 to 950 ° C. at a temperature rising rate of 10 ° C./second or more;
A temperature holding step of holding the maximum heating temperature T ° C. in (40−T / 25) seconds or less;
The manufacturing method of the steel plate member which gave the hot press process which has a cooling process cooled while pressing to the Ms point or less which is the formation temperature of a martensite phase at the cooling rate of 1.0 degree-C / sec or more from the said highest heating temperature T degreeC.
The method for manufacturing a hot-pressed steel plate member according to claim 4, wherein the steel plate member contains at least one of B, Ti, Nb, and Zr in a content of 0.1 wt% or less.
6. The method for manufacturing a steel plate member subjected to hot pressing according to claim 4 or 5, wherein, during the cooling step, press processing for forming the steel plate member into a predetermined shape is performed at least once before reaching the Ms point. .
The method for manufacturing a steel plate member subjected to hot pressing according to any one of claims 4 to 6, wherein the steel plate member is cold-rolled at a rolling rate of 30% or more before the heating step.